Conventionally, there is available a technique (Tissue Harmonic Imaging to be referred to as a THI hereinafter) which images a second-order harmonic component (Tissue Harmonic) contained in an echo signal from a living tissue of an object. A second-order harmonic component is proportional to the square of a sound pressure. For this reason, THI can generate an ultrasonic image with reduced artifacts such as sidelobes.
Scanning a region exhibiting little attenuation of ultrasonic waves may display even weak reflected echoes such as blood flow echoes (to be referred to as spontaneous echoes hereinafter) in the ultrasonic image generated by THI. Displaying such spontaneous echoes makes it difficult for the operator to observe an ultrasonic image. If, for example, spontaneous echoes are reflected echoes from blood and have high intensity, the above region is determined as a living tissue of the object. As a consequence, no color image may be displayed in a region which is ought to be a blood flow region.